Continuing advances in information handling technology have led to the requirement for data storage and retrieval systems capable of processing extremely large volumes of information. A proposed method for satisfying this demand is that of optical recording, and in particular optical disk recording, which allows recording and accessing of information at very high data rates with a much greater recording density and archivability than is possible with available magnetic recording systems.
A highly focussed laser beam is used to record and recover the information on the optical recording media. The selection and alignment of diode lasers into an optical recording system is described by Bartolini et al in IEEE Journal of Quantum Electronics, p.69, 1981, and both read and write apparatus are disclosed in British Patent Application No. 2 016 747A.
Several different systems have been examined in an attempt to develop the ideal laser recording system, although, the use of recording media comprising thin metal films, as disclosed by Lou et al, J.Vac.Sci.Technol., 18, p.78, (1981), is currently the system which is most widely commercially available. However, optical recording media comprising a recording layer of an organic dye are much less expensive to manufacture and are increasingly more attractive, providing suitable materials can be found. Additionally organic dye based optical recording systems do not constitute as significant an environmental hazard as certain metal based systems.
As well as providing advantages of cost and low toxicity, the thermal properties of organic compounds are generally superior to those of metals since they possess lower thermal conductivities and more appropriate melting/decomposition temperatures. In dye based optical recording systems it is important that the absorption/reflection spectrum of the dye corresponds as closely as possible with the emission of the recording laser. Of the various lasers available, semi-conductor laser diodes have advantages over conventional gas lasers, of low cost and compact size, and possibility of signal modulation. The problem is, therefore, one of finding organic materials which have all the requisite physical properties and which absorb strongly in the region compatible with laser diodes, the majority of which emit in the near infrared between 800 and 850nm.
The preferred method of fabricating optical recording media utilising an organic dye as the light absorber is by a solvent coating technique such as spin coating and accordingly suitable dyes must possess some solubility in suitable solvents.
In one construction known as air incident (AI), the dye layer is coated onto a reflectorized substrate. Typically, the reflectivity is provided by an evaporated film of a metal such as aluminium. During recording, portions of the dye layer are ablated by an air-incident intensity modulated laser beam, thereby exposing portions of the reflective metal film and recording data as a reflectivity pattern.
The current trend within the optical recording industry is to move toward an air sandwich optical disk construction, as described by G. Bouwhuis et al in Principles of Optical Disc Systems, pub. Adam Hilger Ltd, pp. 213-214, (1985). In this type of construction the laser beam passes through the substrate before striking the dye layer, a construction commonly referred to as substrate incident (SI).
Several classes of organic dyes have been proposed for use in optical recording media of the types described above and are discussed by Kuder in J. Im. Technol., 12, p.140 (1986). However, known dyes have failed to meet all of the strict requirements demanded of optical recording media. The reasons for failure are varied, but include thermal and/or photochemical instability, poor solubility in suitable solvents and low light absorption.
The use of vapour-coated phthalocyanines in optical recording media is known and disclosed, for example, in U.S. Pat. Nos. 4 241 355 and 4 298 975. The dyes claimed in these patents cannot be used to form a solvent-coatable recording media, being insoluble in useful organic solvents.
Hereinafter, the abbreviation PcM is used to denote the phthalocyanine nucleus: ##STR2## in which:
M represents 2 hydrogen atoms, a metal or metal oxide. Of the peripheral carbon atoms available for substitution, substitution positions numbered 1, 4, 5, 8, 9, 12, 13 and 16 are designated the ortho positions, and those positions numbered 2, 3, 6, 7, 10, 11, 14 and 15 the meta positions.
(t-Butyl).sub.1.4 PcVO, a soluble derivative of vanadyl phthalocyanine, is known to precipitate primarily as a non-IR absorbing glassy solid (phase I) when spin-coated with a vinyl chloride-vinyl acetate copolymer from 1,1,2-trichloro ethane (Law, J. Phys. chem., 89 p.2652 (1985). It has been found that exposure of this solid to a solvent vapour, preferably ethyl acetate, results in crystallization inside the polymer matrix to an IR-absorbing phase II, see U.S. Pat. No. 4 529 688.
German Patent Application No. 3446418 discloses an optical recording medium comprising a substrate supporting a light-absorbing layer containing an amino-substituted phthalocyanine dye which has a strong absorption between 650 and 800 nm, and which can be coated either from solution or by vacuum deposition. It has been found that many amino-substituted phthalocyanines invariably have poor thermal stability.
European Patent Application No. 0 186 404 discloses an optical recording medium comprising a substrate supporting an optically deformable film containing a phthalocyanine dye in which at least 5, and preferably 8, of the peripheral carbon atoms in the ortho positions of the phthalocyanine nucleus are linked by a Group V B or Group VI B atom to a carbon atom of an organic radical.
Japanese Patent No. 61-246091 broadly discloses an optical recording medium having a recording layer of a phthalocyanine dye in which between 1 and 16 of the peripheral carbon atoms are linked by nitrogen or a Group VI B atom to an organic radical. The preferred number of substituents is stated to be between 3 and 16 although in all of the optical recording media disclosed the dyes possess at least 7 substituents.
Japanese Patent Application No. 63-149189 discloses an optical recording medium comprising a substrate supporting a recording layer comprising a class of phthalocyanine dyes, in which at least 3 to a maximum of 8 of the peripheral carbon atoms bear a substituent. From 1 to 4 of these substituents are organic radicals bonded through a group VB or VIB atom, while the remainder (if any) are organic radicals bonded through carbon. The substitution patterns are undefined, except that it is preferred that substituents of a particular type are in an uneven distribution around the periphery. Thus, the compound ##STR3## is disclosed, but the position of the arylthio substituents is not defined. The wavelength of maximum absorption (.lambda.max) for this compound is quoted as 818 nm.
Japanese Patent No. 63-312888 discloses an optical recording element comprising a substrate supporting a recording layer comprising a class of phthalocyanine dyes, in which 4 of the peripheral carbon atoms in the meta positions of the phthalocyanine nucleus are linked by a S atom to a carbon atom of a C.sub.1-20 straight chain alkyl group.
There has now been found a further class of substituted phthalocyanine dyes which possess particularly desirable properties for use in optical recording media.